A TMR element (a tunnel magnetoresistive element) that generates resistance change in response to an applied magnetic field using a tunnel effect has recently come into use as a magnetoresistive element for detecting irregularities on a magnetic moving body. Here, the TMR element is formed with a pinned layer constituted by a ferromagnet and having a fixed magnetization direction, a tunnel film constituted by an insulating layer, and a free layer constituted by a ferromagnet and having a magnetization direction that varies freely in response to an external magnetic field.
FIG. 44 is a view showing an R-H characteristic of a TMR element. More specifically, FIG. 44 shows resistance change occurring when an external magnetic field parallel to the magnetization direction of the pinned layer of the TMR element is applied. When a parallel magnetic field of a certain value or more is applied in the magnetization direction of the pinned layer, as shown in FIG. 44, a resistance value of the TMR changes to a minimum and is thus saturated. Further, when an antiparallel magnetic field of a certain value or more is applied, the resistance value of the TMR changes to a maximum and is thus saturated. FIG. 45 is a view showing an R-θ characteristic of the TMR element. More specifically, FIG. 45 shows resistance change occurring when the magnetic field that saturates the resistance value in FIG. 44 is applied to the TMR element and an angle thereof relative to the magnetization direction of the pinned layer is varied. As shown in FIG. 45, when 360 deg is set as a single period, the resistance value changes in accordance with an angle formed by the magnetization direction of the pinned layer and the direction of the applied magnetic field (the magnetization direction of the free layer).
As is evident from FIG. 45, with regard to a magnetic detection apparatus that uses resistance value change corresponding to variation in the angle formed by the pinned layer magnetization direction and the free layer magnetization direction, use is preferred with an angle in the vicinity of 90 deg, where the resistance change corresponding to the formed angle is greatest. A direction of a bias magnetic field applied to the TMR element at or above the saturation magnetic field (i.e. the magnetization direction of the free layer) is therefore set in the vicinity of 90 deg relative to the magnetization direction of the pinned layer.
FIG. 46 is a view showing a magnetic circuit configuration of a conventional magnetic detection apparatus employing a TMR element. Further, FIG. 47 is a partially enlarged view of the magnetic circuit configuration shown in FIG. 46. FIG. 46 shows a TMR element 1, a magnet 2, a magnetic moving body 3 shaped so as to induce magnetic field change, and a rotary shaft 4. When the rotary shaft 4 rotates, the magnetic moving body 3 rotates synchronously therewith. FIG. 47 also shows an IC 5 including a circuit board or a processing circuit on which the TMR element is mounted.
When the magnetization direction of the pinned layer of the TMR element 1 is set as an X axis direction, a direction perpendicular to the X axis direction and perpendicular to a pinned layer plane is set as a Z axis direction, and a direction perpendicular to an XZ plane is set as a Y axis direction, the magnetic moving body 3 is disposed on one side of the TMR element 1 in the Y axis direction so as to move substantially parallel to the X axis direction. The magnet 2 is disposed on an opposite side of the TMR element 1 to the magnetic moving body 3 in the Y axis direction. The magnet 2 is polarized in the Y axis direction so as to apply a magnetic field to the TMR element at an angle of substantially 90 deg relative to the magnetization direction of the pinned layer of the TMR element 1.
FIG. 48 is an illustrative view relating to the angle of the magnetic field applied to the TMR element by the conventional magnetic detection apparatus, and shows the angle of the magnetic field applied to the TMR element when the magnetic moving body 3 rotates. FIG. 48A shows a point at which one side of a projection on the magnetic moving body 3, in the X axis direction, is close to the TMR element, FIG. 48B shows a point at which the projection on the magnetic moving body 3 opposes the TMR element in the Y axis direction, and FIG. 48C shows a point at which the other side of the projection on the magnetic moving body 3, in the X axis direction, is close to the TMR element.
FIG. 49 is a view showing variation in the angle of the magnetic field applied to the TMR element by the conventional magnetic detection apparatus. More specifically, FIG. 49 shows a result of a magnetic field simulation simulating variation in the angle of the magnetic field applied to the TMR element corresponding to irregularities appearing as the magnetic moving body 3 rotates. Further, FIG. 50 is a view showing the R-θ characteristic of the TMR element provided in the conventional magnetic detection apparatus, and shows resistance change generated in response to variation in the angle of the applied magnetic field, as shown in FIG. 49. Furthermore, FIG. 51 is a view showing resistance change in the TMR element provided in the conventional magnetic detection apparatus. More specifically, FIG. 51 shows resistance change in the TMR element corresponding to the irregularities on the magnetic moving body 3, which is obtained by varying the angle of the applied magnetic field as shown in FIG. 49.
Further, FIG. 52 is a view of a circuit used in the conventional magnetic detection apparatus to process the resistance change occurring in the TMR element. FIG. 53 is a view showing an operation waveform generated by the conventional magnetic detection apparatus, and shows a differential output voltage and a final output corresponding to the irregularities on the magnetic moving body 3, which are obtained by the circuit shown in FIG. 52. The processing circuit shown in FIG. 52 is capable of generating a final output signal that reverses at a projecting portion center and a recessed portion center of the magnetic moving body 3 by comparing the differential output voltage shown in FIG. 53 with a reference voltage Vref (see PTL 1, for example).